CHAPTER V. 



CLEAVAGE (SEGMENTATION) . 



Following fertilization and the commingling of male and female chromo- 

 somes, there occurs the usual longitudinal splitting of these chromosomes as in 

 ordinary mitosis. One-half of each chromosome now passes toward each 

 centrosome. The result is that one-half of each male chromosome and one- 

 half of each female chromosome enter into the formation of each of the two 

 new daughter nuclei (Fig. 22, 4, 5 and 6). The phenomena which follow are 

 apparently identical with those of ordinary mitosis and result in two similar 

 daughter cells. Each of the latter next undergoes mitotic division. In this 

 manner are formed four cells, eight cells, sixteen cells, and so on. This early 

 multiplication of cells which follows fertilization is known as cleavage or seg- 

 mentation of the ovum, the cells themselves are known as Uastomeres and the 

 cell mass as ihe^momla. 



Important differences occur in the cleavage of eggs of different forms of 

 animals, due in large measure to the mechanical factors incident to variations 

 in the amount of yolk and its distribution within the egg. Upon this basis the 

 following classification of the forms of cleavage has been made. 



FORMS OF CLEAVAGE. 



a. Equal e.g., meiolecithal eggs of 



Sponges, Echinoderms, some 

 Annelids, some Crustaceans, 

 some Mollusks, Amphioxus, 

 Mammals. 



b. Unequal e.g., mesolecithal eggs of 



Cyclostomes, Ganoid Fishes, 

 Ainphibians; usual type in 

 Annelids and Mollusks. 



a. Superficial e.g., centrolecithal eggs 



of Arthropods. 



b. Discoidal e.g., polylecithal eggs of 



Cephalopods, Bony Fishes, 

 Reptiles, Birds. 

 40 



Holoblastic (complete or total) 



Meroblastic (incomplete or partial) 



